Description of the Related Art
Stereoscopic systems provide a viewer with a three-dimensional representation of a scene (or an object), using two or more, two-dimensional representations of the scene. The two-dimensional representations of the scene are taken from slightly different angles. The goal of stereoscopic systems is to produce one or more binocular views of a scene to the viewer. A full-parallax view accurately simulates depth perception irrespective of the viewer""s motion, as it would exist when the viewer observes a real scene.
Stereoscopic systems include autostereoscopic systems and non-autostereoscopic systems. Non-autostereoscopic systems require a viewer to use a device, such as viewing glasses, to observe the three-dimensional view, while the three-dimensional effect of autostereoscopic systems may be observed by viewing the system directly.
Early stereoscopic devices used prismatic, total internal reflection (TIR) to simultaneously present two views of a scene, such as the Swan Cube. Prismatic TIR allowed the views to be presented to the viewer such that each of the viewer""s eyes was presented one of the two images, thus creating a perception of depth. Prismatic devices simulate depth perception for only a single viewing angle.
After the introduction of transparent plastic optics, autostereoscopic devices using one-dimensional arrays of cylindrical lenses (known as lenticular lenses) were created. A lenticular lens array has an associated array of composite strip images. Each lenticular lens presents the viewer a selected portion of its strip image such that the combined presentation of all of the lenticular lenses presents a three-dimensional view of the scene.
Devices using lenticular lenses have several shortcomings. First, because the lenticular lenses are cylindrical (i.e., they have optical power in a single dimension), they produce parallax only on a horizontal viewing axis. If the viewer""s viewing angle departs from the horizontal viewing axis, the three-dimensional representation ceases to exist. Second, the lenticular lenses are highly astigmatic, and therefore, the viewer cannot bring the three-dimensional representation fully into focus. Third, if the two-dimensional images require illumination through the lenticular arrays (i.e., the images are not self-radiant, or the images are not printed on a transparent or translucent material that is capable of backlighting), the three-dimensional presentation will have uneven radiance resulting from uneven distribution within the array.
Another autostereoscopic system uses an array of spherical (or aspherical) lenses. Spherical lens array systems have an associated two-dimensional array of microimages. Each microimage is a two-dimensional view of a scene, captured from a slightly different angle. Unlike lenticular lenses, spherical lenses have optical power in two dimensions, thus allowing the viewer to maintain a three-dimensional representation of a scene despite departing from the horizontal viewing axis.
Each spherical lens presents the viewer a selected portion of a corresponding microimage such that the combined presentation of all of the spherical lenses presents a three-dimensional view of a scene. An advantage of spherical (or aspherical) arrays of lenses is their ability to capture arrays of microimages for use with three-dimensional viewing systems. The process of capturing arrays of microimages is known as integral imaging. An image captured by a spherical lens array is initially pseudoscopic, but may be made orthoscopic by reproduction of a captured image using a second array.
The shortcomings of spherical arrays have included that lenses in a lens arrays have excessive aberrations and a tendency to transmit light from multiple microimages. Both of these shortcomings have resulted in reduced image quality.
A difficulty encountered in capturing and reproducing images is optical crosstalk between lens systems of the array. Crosstalk causes overlap of adjacent images, resulting in degradation of the microimages. Solutions to crosstalk have ranged from modifications of the scene when creating the microimages, to optomechanical modifications of the lens arrays. Optomechanical modifications of the lens arrays have included baffles that limit the field of the lens systems comprising a lens array. The baffled lens systems are said to be field-limited. And a field-limited system whose field does not overlap the field of adjacent lens systems is said to be xe2x80x9cisolated.xe2x80x9d Solutions to crosstalk have been costly to implement.
In general, it has been found that the above recited optical goals may be achieved by the use of two arrays, the two arrays typically including three active optical boundaries and a planar surface disposed at the focal length of the array. In the invention, a first boundary is convex and substantially spherical, a second boundary is concave and usually oblate, and a third boundary is prolate and predominantly convex. The internal surfaces of the second and third optical boundaries may, in some embodiments, also be employed as efficient angle-selective reflectors. In preferred embodiments of the invention, the two array layers are joined using bonding agents of specific optical characteristics. The two preferred compositions are 1) a light-absorbing bonding agent, and 2) a low-index UV-curing fluoropolymer. Two embodiments of the microlens may be conceived of as roughly analogous to, in the first case, an aspheric doublet with an internal aperture, and in the second case, an aspheric cemented triplet.
The invention includes arrays in which the outer spherical lens surfaces are imbricated, i.e. having a polygonal aspect when viewed on-axis, as well as embodiments in which the convex outer lens surfaces are formed as independent features on a continuous planar background, as suits their application. Other specific embodiments will be understood by way of the following figures and description.
The improvement of the array optics generally relies on a low-index region of highly aspheric geometry located within an array of higher-index optical material, such as acrylic, polycarbonate or polyetherimide. The region is typically created by permanently bonding two discrete arrays such that the low-index material is entrapped between the bonded arrays. Each microlens cell therefore has three significant optical boundaries, and a continuous planar rear surface. The general family of geometries illustrated in this document have been found to greatly reduce spherical aberration, while also eliminating the preponderance of field curvature. Because the performance of these arrays is not implicitly degraded at short focal lengths, a continuous, full-parallax image field of 60xc2x0 or more can be attained.
The arrays generally fall into two categories. The first type of array uses air as the low-index material. The distinctive equiangular geometry of these arrays allows the optical pathway to be confined to a central angular zone; because of this ability to restrict light, these arrays will be referred to as field-limited arrays. This type of array may be used, for example, in illuminated displays electronic image detection, machine vision, and real-time 3D video capture. A second type of array uses a fluoropolymer as the low-index material, and conveys a great preponderance all incident light to the image plane. These arrays are referred to as open-field arrays, and may be used to produce high-brightness images for viewing under arbitrary ambient illumination, or for imaging lenses with increased optical gain or reduced lens flare.
By exploiting TIR, the field-limited arrays can be fashioned to eliminate image overlap and confine the converged real microimages to the region of the focal plane immediately associated with their respective microlenses. This light restriction on acceptance allows a set of coplanar microimages to be captured without significant overlap. The array may be used to eliminate the formation of parasitic latent images in an emulsion system, or to prevent analogous optoelectronic noise in photoelectric capture devices.
Additionally, during the transmission of light from a the graphic surface of a field-limited master image to a duplicate, TIR can again be exploited to restrict light production from the master to the same angular range. This light restriction on emission from the array allows printed, developed, or electronically produced images to be optically conveyed through a second array for electronic, electrostatic, or photographic reproduction.
Because both the ingress and egress of light is restricted by the originating field-limited array, an open-field array can be used as the recipient material. The recipient material therefore needs no intrinsic light-restricting processes of its own, which would inevitably tend to reduce the brightness of a passively lit image. Pseudoscopy is inherently rectified by this reproduction process. Bright, highly realistic duplicate images may therefore be reproduced by face-to-face contact printing, ideally with the microlenses in alignment. By logical extension, it may be understood that a preexisting spherical-lens image, of either the field-limited or open-field type, may be reimaged or recorded by a properly arranged field-limited capture device.
An advantage of the invention is that a superior effect can be obtained using readily available optical materials and existing graphic media. The designs shown are based on the use of thermoplastic polycarbonate (xcex7=1.586) for the two component arrays, and either air or a low-index (xcex7=1.360) UV-curing fluoropolymer for the enclosed low-index zones. A modified hexagonal microimage layout which reconciles the hexagonal geometry with digital processes. Following is an outline of general issues that may influence microlens designs formed according to the invention.
Full-parallax Imaging
Although the basic sectional optical geometry may be applied equally to cylindrical-lens arrays, the invention is particularly suited to xe2x80x9cglobalxe2x80x9d imaging, i.e., systems using radially synmetrical microlenses; a distinguishing characteristic of global imaging systems is that they can reproduce parallax on all axes, rather than on the horizontal axis only. Radially-symmetrical elements would also avoid certain aberrations intrinsic to the asymmetrical geometry of cylindrical arrays. Radially-symmetrical systems can more directly capture and reproduce three-dimensional image data. Full-parallax images can include an animation on the vertical axis without introducing binocular error. Wireframe geometries are also more readily and accurately extracted from and reproduced to full-parallax image sets.
Aperture Stops
The ideal aperture-stop location in field-limited arrays is informed by the scale of the microlenses, the mode of their use, and the simulated distance of objects in the scene. In directly-viewed displays, the second refractive surface is generally stopped. In arrays used as masters, the first surface will generally be stopped if the goal is a 1:1 mapping of every graphic element. This ensures that there will be no information shared by adjoining lens cells. When an inner stop is used in a mastering process, information from each master lens will be shared by more than one cell in the duplicate array. Prior to assembly of the field-limited arrays, light-blocking material can be applied to raised areas of the internal surfaces, without risking contamination of the optical surfaces. Total internal reflection and light-absorbing material at the lens interstices than act cooperatively as a comprehensive field-stop.
Total Internal Reflection
TIR is a prominent consideration in diverse processes in the system. The convergent and field-flattening requirements of the design leave few further degrees of freedom in the configuration of the core optics. However, in field-limited arrays, where the entrapped low-index material is air, an important coincident effect arises due to the novel concave geometries of the internal boundaries: each of the two internal optical surfaces is, independently, in a highly equiangular relationship with significant ray sets at critical locations. The arrays therefore restrict light arriving on or emerging from the array to substantially the same central angular field; light beyond that field is deflected by TIR. The nearly equiangular relationship causes the optical pathway to be abruptly cut off at a predetermined angle of incidence, with the normally transparent optical boundary itself essentially acting as a field-stop.
Because total reflection occurs almost simultaneously across the surfaces, an intensity plot taken across a test microimage converged from an image field contrived to have a continuous luminosity will closely conform to standard transmittance curves. Accordingly, a plotting of the irradiance of microimages captured in this manner is therefore substantially flat over about 65% of the absolute diameter of the converged image, dropping off abruptly thereafter.
The restriction of light entering the array largely prevents the overlap of microimages, permitting a continuous detector field to be employed without internal barriers between the individual lens cells. The restriction of light exiting the array to a closely matched angular field creates a reciprocal optical pipeline of virtually identical operational characteristics. In field-limited arrays, extraneous light bypassing the core optics and falling into the lens interstices would typically be blocked by light-absorbing cement at the arrays"" internal contact surfaces.
Some light emanating from the image field is recaptured and returned, via TIR, to the rear surface. Other effects that may exploit partial or total reflection include internal diffusion via prismatic surface reflection, and, in backlit, field-limited displays, the veiling of the image beyond the central viewing zone.
Vergence Angles
The eye adapts to focus objects at various distances. The small angle formed by a vertex on the subject and points on the rim of the pupil is the vergence angle. The lenses may be designed to conform to this geometry so that the pupil is filled at he target viewing distance. This geometry varies according to light conditions as well as distance. Two solutions can generally be found that accurately fill the eye at a given distance from the image, one ahead of the image plane and one behind.
Transmission of Internal Illumination
In images intended to be viewed in under ambient illumination, it is desirable to minimize internal reflection and maximize transmission at the second surface, since an acceptable brightness level in the observed image depends on the very stray light which is reflected away in field-limited arrays. The open-field arrays, which use fluoropolymer as a low-index material, have a geometry that is analogous to that of field-limited arrays, and which provides a similar corrective effect. In contrast, however, the open-field arrays designs permit a great preponderance of incident ambient light to be transmitted to the image plane. Surface reflections at air/polymer boundaries can cause haziness in the viewed image, especially when the image is lit by ambient light. The reduced effective refractive index at the air/fluoropolymer boundaries greatly diminishes any such haziness.
Diffusion of Internal Illumination
Interstitial regions are the zones between the active lens surfaces. Because the active area of the internal lens surfaces is considerably less than the hemispheric diameter of the outer lenses, the extent of internal lens interstices can be considerable, even when the fill rate of the outer array approaches 100%.
Images viewed in reflection, i.e., like conventional printed images and photographs, depend on a wash of ambient light over the surface of the graphic material for even illumination. The refractive nature of arrays therefore significantly alters the illumination characteristics of the graphic plane. The convergent nature of refractive arrays of course persists, even when the array is only used for viewing; the core optics of the array converges incident light, arriving from the ambient scene in which the viewer is located, into inverted real microimages that superimpose themselves on the upright graphic microimages.
Light arriving this central zone is therefore of diminished utility, since the observers sight-lines will by definition be aligned with the inverted image his or her own eyes, and not therefore with any bright ambient-scene light source imaged by the microlens. An emulation of the wash of light experienced in a conventional image can only be provided by light passing through lens interstices from coordinates in the ambient field outside the highly focal central angular zone.
The design goals of the core optics and of the interstitial optics are therefore substantially opposite: ideally, light that traverses the array via the core optics should be perfectly convergent at the image plane, and light that passes through the interstices should be perfectly diffused.
Hybrid Achromatization
Another incidental benefit of the corrected designs is that, within each of the arrays described, a surface is located with its vertex in the vicinity of the center of curvature of the outer optical boundary. This allows the effective use of diffractive achromatization, which is traditionally precluded in such wide-angle arrays, due to decentration of the corrective effect. The inclusion of a corrective hybrid surface would improve the focal acuity of the array, reduce its thickness, and would slightly enhance the angular restriction outlined above. Knife-edge features would be carried internally and would thus be protected from degradation.
Antireflective Microstructures
Performance would be further optimized and economized by the use of hybrid refractive/diffractive surfaces and anti-reflective xe2x80x9cmoth-eyexe2x80x9d microstructures. In production, AR relief structures would typically be more cost-effective than coating, and would improve the saturation and contrast of the observed image. The enclosed internal optical surfaces could use high aspect-ratio subwavelength AR microstructures which might be subject to degradation if formed on an external surface. The outer array surface might be formed with a similar but more robust AR microstructure topology.
Techniques for producing an index gradient in polymers are well-known, and graded internal boundaries would increase transmission of ambient light to the graphic image plane in open-field arrays. Because most display applications require cost-effective replication in high volumes, however, AR microstructures would commonly provide a cost-effective emulation.
Image Reproduction
Conventional direct reproduction processes would commonly be employed. However, the improved optics suggests the potential of alternate methods.
The field-limited array possesses the optical characteristics necessary for capturing or reproducing an image with a restricted angular field; the fluoropolymer-filled array provides an array with excellent viewing qualities. An implication of this pairing is that, given two arrays of sufficient acuity, light may be passed from a pseudoscopic master to a duplicate, intrinsically rectifying the pseudoscopic spatial inversion in the process. This technique may provide new opportunities for image correction and for high-volume, high resolution image reproduction. Photographic 3D images can, by this process, be reproduced at industrial rates in a manner analogous to conventional contact prints. Registration requirements can be relaxed due to the intrinsic angular identity of the graphic elements.
Although the lower limit of the lens size is practically limited by diffraction, this process could also, on a certain scale, be used to directly capture and seamlessly reproduce the 3D attributes of a real scene. More commonly, images would be digitally processed and mastered using microlens arrays which would be so fine as to be impractical for direct capture.
Array Assembly
In addition to the diffusers, the interstitial zones normally include mating relief features to promote alignment and adhesion. The draft angle of the contact surfaces would cause the two arrays to self-align. The mating features would also ensure that the microlenses were accurately aligned and seated, and could eliminate the need to monitor calibration of the arrays during continuous production. Volumes defined by surface features may be used to precisely meter adhesives and other fluid materials, such as the fluoropolymer used in open-field arrays.
Microimage Layout
A hexagonal lens layout allows microimages of various proportions to be efficiently tiled on the focal plane. The horizontal range can be extended using an irregular microimage profile so that a bias is given to the horizontal axis. The bias is sufficient to accommodate the added dimension demanded by binocular vision, without reduction of the vertical range. A hexagon is used which, having been derived from a square, rectifies a hexagonal layout with orthogonal digital rasterizations.
Image Processing
Images mastered by a film recorder can computationally compensate for angular reflection losses at the image perimeter. The white-point of the microimages may be set lower at the center of the microimage than it is at its perimeter, so that during reproduction to a second array the relative irradiation distribution of the illuminated images varies in a radial manner and in inverse proportion to the calculated total system losses between the master microimage plane and the eventual viewer. Compensated losses could be due to reflection, absorption, vignetting, or to inconsistency of source illumination; all these effects would generally tend to be expressed at more oblique angles of incidence, so computational or physical filters would reduce transmission in the axial zones of the microimages in a continuous gradient from the center point of the microimage.
Image processing can be integrated on-board a CMOS detector before digital image data is written to a storage device, or may be implemented after recording. Pseudoscopy might electively be corrected on-the-fly during image capture. Digital filtering can also equalize image intensity, contrast, and color attributes and can detect and eliminate artifacts from remaining microimage overlap. These processes can increase the useful area of a detector, whether emulsion-based or electronic.
The invention therefore includes diverse embodiments derived from the structures and processes generally noted above.
A first embodiment of the invention therefore results from an understanding that, in a positive lens having a convex spherical front surface, in which the thickness of the lens approaches or surpasses the radius of curvature of the lens, a second surface may be devised so that, for each ray converged by the first surface from a collimated incident ray set, and for a particular predetermined incident angle of light impinging on the first surface, the convergent cone of rays can be made to arrive on the second surface so that the critical angle is encountered virtually simultaneously by all rays in the convergent set. Light transmitted and converged by the first convex spherical surface from beyond a certain incident angle, typically near the critical angle of the optical material, is wholly prevented from passing through the second surface, and instead reflected away by total internal reflection. In contrast, rays convergent about the optic axis are nearly normal to the second surface, and therefore pass with only a small percentage of their light reflected. As a result, the image field is confined to a radius on the image plane directly associated with the refractive index of the material, and the light distribution across the image field closely parallels the transmittance curves for the refractive medium. In general, radially symmetrical surfaces for the second surface that approximate the ideal equiangular state will be negative in curvature, oblate, and located in the vicinity of the center of curvature of the primary convex spherical surface. Solutions for the third surface are all predominantly positive in curvature, but the curves specified are idiosyncratic according to the problem. This surface may therefore be oblate, spherical, or prolate, or reflexed according to the demands of the individual system.
A second embodiment of the invention is derived from an understanding that the surface curvature implied by the first embodiment may be coincidentally employed to correct spherical aberration.
A third embodiment of the invention stems from an understanding that the absolute perimeter of the image field can be further reduced by including a third refractive surface which is predominantly positive in refractive power,
A fourth embodiment of the invention stems from an understanding that if that third refractive surface is located at a specific distance from the second refractive surface, a substantially flat focal field will result.
A fifth embodiment of the invention stems from an understanding that if that third refractive surface is additionally substantially prolate in geometry, spherical aberration may also be effectively corrected across the image field.
A sixth embodiment of the invention stems from an understanding that if the region between the second and third optical surfaces contains a material having a refractive index near 1, that light both received by and reproduced from the array may be confined to substantially the same central angular range.
A seventh embodiment of the invention depends on a recognition that in microlens images viewed in reflection, e.g. a print or photograph, apparent image brightness depends in large part on the angular composition of the light arriving on each microimage plane from adjacent cells, rather than via the lens cell associated with the immediate region of the graphic surface. Therefore, internal interstices may be expressly made transparent and diffusive in reflective devices to optimize internal distribution available illumination.
An eighth embodiment of the invention stems from an understanding that if the region between the second and third optical boundaries contains an optically transparent material having a refractive index greater than 1, but substantially less than that of the primary array material, and especially if the second refractive surface is located with its vertex in the vicinity of the center-of-curvature of the primary spherical lens, that the system may be provided with a correction for field curvature and spherical aberration, while at the same time maximizing the transmission of ambient light to the image plane by deterring losses by partial or total reflection at the second refractive surface.
A ninth embodiment includes a positive power diffractive element on one of the internal surfaces, so that the system may be achromatized, so that the system""s converging power and viewing range may be increased, and so that the diameter of the systems image fields may be electively minimized.
A tenth embodiment of the invention includes mating relief elements which assist in the alignment and spacing of the optical surfaces.
An eleventh embodiment is directed at the optical reproduction of an original planar image to a second planar image surface, and generally relies on the conjoint operation of two of the bonded arrays, i.e. four arrays total, of similar pitch, having their lenticulated sides facing and in registration. It is understood that this may be implemented as a linear scanning device or as a flatbed reimaging device for conventional two-dimensional media. It is also understood that the one of the bonded arrays may be permanently affixed to the two-dimensional graphic material of the original planar image, whereby an illusionistic three-dimensional image may also be provided with a means by which it may be electronically reimaged for storage, manipulation, or duplication.
A twelfth embodiment describes a microlens devised for use as a surface-mount imaging lens wherein the optics may be entirely composed of transparent material, and wherein a wide-angle, high-acuity image-forming and mounting system may be provided with as few as two individual parts.
A reproduction system in accordance with the invention comprises a first array of lenses comprising a first plurality of lens systems, the first array of lenses having a object plane, each of the first plurality of lens systems having an optical axis, and a second array of lenses comprising a second plurality of lenses, the second array of lenses having an image plane, each of the second plurality of lenses having an optical axis, the second array of lenses optically coupled to one lens system of the first array of lenses. The at least one lens system of the lens systems of the first plurality of lenses, or one lens systems of the second plurality of lenses, is an optically field-limited surface for one of the image plane and the object plane.
A microimage capture system in accordance with the invention for capturing a plurality of microimages of an object comprises a photosensitive medium, and a lens array optically coupled to the photosensitive medium. The lens system comprises a first convex surface having a radius of curvature R, and a first concave surface optically coupled to the first convex surface via a first material, the first concave surface positioned a distance substantially equal to R from the first convex surface. The microimage system further comprises a second convex surface optically coupled to the first concave surface via a material having a lower index of refraction than the first material, the second convex surface positioned at least a distance equal to 0.05 R from the first concave surface.
A three-dimensional viewing system for producing a three dimensional representation, comprising a plurality of microimages and a lens array, optically coupled to the plurality of microimages. The lens array comprising a first convex surface having a radius of curvature R, a first concave surface optically coupled to the first convex surface via a first material, the first concave surface positioned a distance substantially equal to R from the first convex surface, and a second convex surface optically coupled to the first concave surface via a material having a lower index of refraction than the first material, the second convex surface positioned at least a distance equal to 0.05 R from the first concave surface.
An optical system in accordance with the invention comprises a lens system, and a photic element optically coupled to the lens system. The photic element comprises a first area symmetric about a first axis, having a dimension X along the first axis, and a dimension Y along a second axis, and a second area contiguous with the first area and symmetric about the second axis having a dimension A along the first axis and a dimension B along the second axis. The dimension A is smaller than dimension X.